The blades, notably of a fan, but also of a low-pressure compressor, made of composite material with carbon fibers are made in different ways. According to one manufacturing method, a stack of one-way plies or preimpregnated wovens is produced that is placed in a mold while orienting the successive plies differently before compacting and polymerization in an autoclave. According to another method, woven preforms of dry fibers are prepared that are assembled by sewing or else a single preform made of woven fibers in three dimensions, that is impregnated with resin by injection in a closed mold. The blade is formed in a single piece comprising the root with the airfoil. It comprises various protections in order to reinforce the thermomechanical resistance thereof. A metallic protection is therefore attached to the leading edge or to the whole contour of the airfoil comprising the leading edge, the blade tip and the trailing edge, for example in the form of a titanium part bonded over the whole surface of the leading edge and over a front portion of the outer surfaces of the extrados wall by mounting a protective film that can be made of a synthetic material, polyurethane for example, and directly bonded to the intermediate part.
The object of the invention is this type of blade with at least one protection along the leading edge. An example of manufacture is described in patent EP1777063 in the name of the present applicant.
The object of the invention is more generally any type of blade made of composite material, the airfoil of which is formed of fibers, threads or filaments, optionally woven, impregnated with a heat-curable resin.
Flutter is a phenomenon of coupling between the aerodynamic and elastic characteristics of the blade creating unstable situations. A flutter manifests itself asynchronously. Subsonic flutter is distinct from supersonic flutter. The fan blade is mainly concerned by subsonic flutter.
Flutter is a phenomenon that is difficult to predict, because of the complexity of the coupling between the aerodynamic and mechanical responses. Moreover, the mechanical damping of the blade is usually not very well known. Finally, in the current design of increasingly loaded blading, flutter is a phenomenon that must be particularly taken into account.
During the design of the fan blade, a margin of flutter is estimated, which measures, at a given flow rate, the difference between the line of flutter and the line of operation. This value is usually established based on a known reference (the closest) by adding thereto the differences calculated between this reference configuration and the new configuration. The criteria used nowadays for subsonic flutter on 1F and 1T modes and on zero diameter coupled mode are:                Twist Bend Coupling (TBC), representing the ratio between the movements in twisting mode and in bending mode. The higher the TBC parameter, the greater the risk of flutter also.        The reduced speed or Strouhal criterion given by the following formula:VR=W/C*f*pi, where W is the relative speed, C the chord of the blade at a given height and f the frequency of the blade mode in question. This criterion represents the coherence between the vibrational frequency of the blading and the frequency of the unsteadiness of the flow along this blading.        
Other factors may influence the flutter margin and may occasionally be used when the phenomenon is encountered during tests: reduction of the specific flow rate, reduction of the number of blades or increase in the chord, lubrication of the blade root, detuning.
The object of the invention is to improve the harmonic response of the blade to synchronous aerodynamic excitations such as:                inlet duct distortions generated by flying conditions in angle of incidence—climbs, descents, crosswinds,        the harmonic excitations generated by a residual imbalance,        backpressure fluctuations, induced by a fixed impeller of the stator type on a fan impeller,        wake or backpressure fluctuations induced by a moving fan impeller on its neighbor in the case of an architecture with two contrarotating rotors.        